is there any way how to translate 'yii' category with Yii::t() method?
Note, category 'yii' is reserved for Yii framework core code use.
In framework folder I can see some message files for chinese, but if I get the quote above right, it says something like "You cannot translate those messages" :/. Is there any way how to do it w/o annoying class extending and method overwriting to change proper code to show what I want depending on chosen language?
Not a problem. I don't really think you need the translation of those core messages though. I guess you mainly want those form error messages to be in your language. If so, you just need to specify the 'message' option in the validation rules.